Water-cooled automotive-type internal combustion engines (ICEs) usually have a forced pressure pump lubricating circuit, in which oil which has been passed through the engine for lubrication is cooled by being collected in an oil pan, forming an oil sump. The temperature of the cylinder walls of the engine, in water-cooled engines, is roughly uniform and counteracts excessive heating or cooling of the oil. For safety reasons, the oil temperature must be maintained at a value which is somewhat too low for optimum operation. Upon cold-starting an engine, particularly after a vehicle in which the engine has been installed has been parked outdoors in wintertime, and especially during a cold rain, the oil temperature is excessively low. Too low oil temperature results in excess use of fuel, which may reach several percent of excess fuel consumption, and, typically, is in the order of about 3% of excess. Such excessive fuel consumption is due entirely to excessively low lubricating oil temperature. In start-stop operation, and for short-distance runs, upon fractional loading of the engine, and poor environmental and weather conditions, the excess fuel required to overcome frictional losses due to increased oil viscosity rises above the foregoing average excessive values.
It has previously been proposed--see German Patent Disclosure Document DE-OS No. 28 11 144--to provide for thermostatic control of lubricating oil. Such thermostatic control has some disadvantages, however. The thermostatic lubricating oil control cannot be installed in existing engines without major modification; the thermostat has movable parts which may wear or malfunction, so that appropriate operation over the lifetime of the engine cannot be insured. Possible defects in the thermostatic control may not be noticed in time and, therefore, continuous lubrication by oil at excessively low temperature may result in damage to the engine. The arrangement is comparatively expensive in manufacture.
The technical basis on which the system of the referenced Disclosure Document No. 28 11 144 operates considers only temperature. Thus, if the engine has been exposed to extreme cold temperature, and is equipped with cold-temperature special oil, for example of the viscosity class SAE (Society of Automotive Engineers) 10W or 5W-20, the oil may reach dangerously low values of viscosity when thermostatically controlled. In such cases, thermostatic control of the oil is worse than none at all.